Electric current rectifier



Nov. 23, 1954 s. E. MAYER ET AL 2,695;380

ELECTRIC CURRENT RECTIFIER Filed Aug. 17, 1950 Counter E/ectr'oae Inventor 5. E. MAYER HFSTER r. lA/G Attorney United States Patent O ELECTRIC CURRENT RECTIFIER Simon Ernest Mayer and Henley Frank Sterling, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application August 17, 1950, Serial No. 180,100 Claims priority, application Great Britain August 26, 1949 Claims. (Cl. 317238) This invention relates to electric current rectifiers, and more particularly to such rectifiers utillsing the semiconducting properties of a metal oxide and allied materials containing appreciable quantities of a metal oxide.

It has previously been reported that the semi-conducting form of titanium dioxide possesses rectifying properties and a rectifier ratio of about 100 to 1 has been obtained, but only with an internal resistance of the order of megohms in the forward direction (Klarmann, Wiss. Veroff. Siemens, 18, 198 (1939)). It has also been recently stated that current ratios of six to one have been obtained with discs of 3 mm. thickness (Electronic Engineering, October 1946, p. 314.).

It is the object of the present invention to provide commercially practicable rectifiers utilising the above mentioned materials.

The materials to be used in rectifiers according to the present invention owe their semi-conducting properties to a deficiency of oxygen from the stoichiometric proportions, usually too minute to be detected by chemical analysis.

According to the present invention an electric current rectifier comprises a semi-conductor layer and a counterelectrode and a barrier layer therebetween in which the semi-conductor layer and the barrier layer comprise a metal oxide and in which the semi-conductor layer is deficient in oxygen in comparison with the barrier layer.

Another feature of the invention comprises the method of manufacture of an electric current rectifier which comprises reducing the oxygen content of a layer of metal oxide, restoring oxygen to a thin surface layer thereof and applying a counter-electrode to said thin surface layer.

A further feature of the invention comprises the method of manufacture of an electric current rectifier which comprises reducing the oxygen content of a layer of titanium dioxide and depositing thereon titanium dioxide by the decomposition of a compound containing titanium and oxygen and applying a counter-electrode to the deposited layer.

The accompanying drawing shows in section a rectifier according to the invention. In this drawing the thickness of the respective layers has been greatly enlarged in order to show clearly the arrangement. A semi-conductor layer 1, comprising metal oxide such as titanium dioxide, rendered deficient in oxygen in a manner to be later explained, has on one of its surfaces a relatively thin layer 2, comprising metal oxide such as titanium dioxide to which oxygen has been added to overcome any deficiency which might have existed therein. The surface of the layer 2 remote from the layer 1 has thereon a counterelectrode layer 3 which may conveniently comprise a silver paste.

According to the usual rectifier practice contact with the semi-conductor layer may be made by the application of a base electrode.

Some constructions according to the invention will now be described by way of example utilising the semiconducting form of titanium dioxide. A pressed and sintered body of titanium dioxide is formed in the shape of a disc, by making up titanium dioxide in powder form into a paste with water to which has been added some gelatine or glue. The paste is then pressed into the disc form to a density of about 3.5. The disc is then dried, prefired in air to about 1050 C. for about 1 /2 hours. A final firing at about 1300" C. is given in hydrogen for 2,695,380 Patented Nov. 23, 1954 "ice about 1 hour. The disc is then allowed to cool in the furnace in a hydrogen atmosphere.

A base electrode is applied to one face of the disc e. g. by firing a silver paste. The other face of the disc is then polished by known. methods and treated by means of an oxy-gas flame so that the surface attains a temperature of about 500 C. A counterelectrode is then applied to the treated surface. This counterelectrode may be pressed on lead foil, or a silver paste prepared according to specification No. 569.388 so that it can be fired at a lower temperature than 500 C., e. g. 300 C. is applied. Better results than either of these have, however been obtained by the use of a connterelectrode of tellurium. With this last mentioned counterelectrode a typical rectifier of 0.5 cm. give the following characteristics Forward resistance 77 ohms Reverse resistance 90,000 ohms Turnover voltage 20 volts.

} taken at 50 cycles A. C.

made in this manner has been found to have a very high output and moreover the stability and consistency of the rectifier were found to be greatly improved. One such rectifier was found to have the following characteristics:

Forward resistance 1.25 ohms Reverse resistance ohms Turnover voltage 12 volts.

At an operating temperature of 200 C.

The eifective area of the rectifier was of the order of l cm. and a rectifier of this character is eminently suitable for power work.

The surface treatment of the semi-conductor can be given either before or after the application of the base electrode.

The surface treatment may, however, be produced by other means. For example the surface may be treated by application of a chemical oxidising agent, or may be oxidised electrolytically e. g. ,by anodic treatment in sulphuric or oxalic acid. In the case of an electrolytic treatment it is clearly an advantage that this should be carried out after application of the base electrode.

The surface treatment may also consist in the deposition upon the surface of a thin socalled barrier layer of another material of the same type as that of the disc but which contains a higher proportion of oxygen than the body of the disc. For example titanium dioxide de posited from titanium tetrachloride or butyl titanate may be used with a semi-conducting titanium dioxide disc.

It is to be understood that, although discs of titanium dioxide have been described above, the invention is applicable also to rectifiers using as the semi-conductor materials allied to titanium dioxide and containing (whether in mixture or chemical combination) appreciable quantities of that oxide. For example barium titanate or other titanate in semi-conducting form may be used as the semi-conducting body. Or mixed semitaken at 5 0 cycles.

conducting crystals of titanium dioxide and another the decomposition -of buty1 titanate and the thin 'film then heat treated in a hydrogen atmosphere to produce the semi-conducting form of titanium dioxide.

While the 'principlesmf theinvention :have been described above in connection with specific embodiments, andzparticu'lar modifications thereof, it is .to be clearly understood "that this invention is made only by way of example and-not as a'limitation "on the scope of the invention.

What we claim is:

1. An electriccurrent rectifier comprising a layer of titanium dioxide in conducting condition and having a thinsurface layer-'of'titanium dioxide in non-conducting condition, a counte'relectrode on said thin surface layer and a base electrode :of silver paste on the semi-conductor layer on the surface remote fromithe barrier layer.

2. An electric current rectifier' as claimed in claim 1 in which the base electrode and the counter-electrode comprise silver pastes applied by firing at different tern peratures.

3; An electric current rectifier as claimed in claim 1 '4 in which the counter-electrode layer comprises a pressed on lead-foil.

4. An electric current rrectifier as claimed in claim 1 in which the counter-electrode comprises tellurium.

5. An electric current rectifier as claimed in claim 4 in which the counter-electrode contains thallic oxide.

References Cited in the file of this patent STATES PATENTS Number Name Date 879,062 Pierce Feb. 11, 1908 929,582 Garretson July 27, 1909 FOREIGN PATENTS Number Country I Date 483,088 Great Britain Apr. 12, 1938 OTHER REFERENCES JMetal Rectifiers, Henisch (1949),, pp. 127-130. Electronic Engineering, October 1946, pages 313-315. 

1. AN ELECTRIC CURRENT RECTIFIER COMPRISING A LAYER OF TITANIUM DIOXIDE IN CONDUCTING CONDITION AND HAVING A THIN SURFACE LAYER OF TITANIUM DIOXIDE IN NON-CONDUCTIVE CONDITION, A COUNTERELECTRODE ON SAID THIN SURFACE LAYER AND A BASE ELECTRODE OF SILVER PASTE ON THE SEMI-CONDUCTOR LAYER ON THE SURFACE REMOTE FROM THE BARRIER LAYER. 